<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    .bg{
        position:fixed;
        top:0;
        left: 0;
        right: 0;
        bottom: 0;
        background: red;
    }
    </style>
</head>
<body>
    <div class="bg">

    </div>
</body>
</html>